1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to, a method and apparatus for determining frame quality in a mobile communication system, in which frame quality is determined by estimating a received signal to interference ratio (SIR) in case where a power control bit (PCB) for maintaining a call is only transmitted and received under a control hold state in which no data transmission actually occurs.
2. Description of the Related Art
In a mobile communication system using a code division multiple access (CDMA), such as a cellular mobile communication system or a personal communication service system, a plurality of mobile stations transmit and receive frames via a base station or a cell-site. The communication path used by a base station to transmit data frames to a user is called a forward link. Conversely, the reverse link refers to the communication path used by a user to transmit data frames back to a base station.
Each of the frames is composed of symbols encoding a digitized voice and data. These frames may fade during transmission through a multipath because reflections in the transmission environment as the mobile stations move. The frames may also fade by an interference of one specific mobile station with another mobile stations due to a high transmission power.
Also, if a certain mobile station is extremely low in power, the associated base station cannot decode the data of a frame transmitted from the mobile station, resulting in a loss of data. Thus, the base station has to control the frame transmission power of the mobile station to adequately decoding the data of the transmitted frame while minimizing the fading of the transmitted frames.
For frame transmission power control, the base station measures the transmission power of the frame transmitted from the mobile station and sends a power control command to the mobile station according to the measured result. The power control command instructs the mobile station to adjust the transmission power. This power control command is composed of a transmission power increment bit or a transmission power decrement bit for maintenance of an average reception power.
The base station transmits the reception power increment bit or reception power decrement bit from the power control bit with the frequency up-processed user data to the mobile station via traffic channel among the CDMA channels of a forward link.
The CDMA channels in the forward link are generally a pilot channel, a synchronous channel, a paging channel and a traffic channel for forward connection and communication from the base station to the mobile station. The CDMA signals through these CDMA channels are orthogonally processed based upon Walsh function codes and processed as orthogonal pairs of a pseudo noise (PN) sequence based upon a fixed chip rate such that the signals can be code-symbolized.
There are two power control methods.
First, in the closed loop power control, the base station measures a transmission rate-based reception power of a signal received from the mobile station, compares the measured reception power with a predetermined power control reference value and sends an appropriate power control command as a result of the measurement together with user data to the mobile station.
Thus, the mobile station adjusts the transmission power in response to the sent power control command and transmits a signal at the adjusted transmission power.
In the closed loop power control, the power control reference value is determined differently for each mobile stations depending on various parameters such as the moving speeds of the mobile station, the surroundings of the mobile stations, etc. Particularly, the base station adjusts the power control reference value appropriately according to a frame error rate of the received data. Such power control is called an outer loop power control.
As discussed above, the conventional power control of the CDMA mobile communication system is mainly a reverse link control.
But, for a high-speed forward link power control, a mobile station is required to measure the power of a CDMA signal received from a base station and determining which one of the CDMA channels is to be observed and used for the power control.
Meantime, in case of packet data service, data transmission successively occurs while a call is set. In many cases, data transmission discontinuously occurs. Also, although the BS transmits data, data transmission may occur intermittently in a terminal unit. In each case, a medium access control (MAC) layer manages resources using a state-machine to effectively control the capacity of the BS.
In the communication system that provides high rate packet data services, the MAC layer is implemented by an active state, a dormant state, a control hold state, and a suspended state.
The control hold state and the suspended state serve to eliminate interference by an idle user signal that exists in the active state. The control hold state and the suspended state also serve to eliminate overhead generated as a relatively long time is required to transit the dormant state to the active state.
Accordingly, the control hold state and the suspended state exist between the active state and the dormant state. The control hold state means that the PCBs are only transmitted through the traffic channel to maintain a call when no transmission data exists.
Particularly, if transmission data occurs again under the control hold state, the control hold state is transited to the active state so that actual data transmission occurs.
In the communication system that provides high rate packet data services, it is necessary to measure frame quality for outer loop power control for reverse link power control, forward link power control, and channel state monitoring.
However, in case of the communication system that provides high rate packet data services, it is impossible to measure frame quality by the CRC under the control hold state. That is, since the CRC determines whether a receiving frame is poor to measure frame quality, it is impossible to measure frame quality under the control hold state in which no data transmission actually occurs.